1. Field of the Invention
The invention relates generally to glucose monitoring and more specifically to implantable glucose sensors.
2. Background Information
Diabetes is a disease of insufficient blood glucose regulation. In non-diabetic people, the body's beta cells monitor glucose and deliver just the right amount of insulin on a minute-by-minute basis for tissues in the body to uptake the right amount of glucose, keeping blood glucose at healthy levels. In diabetics this healthy regulation system primarily fails due to the following two factors, either alone or in combination: 1) insufficient insulin production and secretion, 2) a lack of normal sensitivity to insulin by the tissues of the body.
The first major breakthrough in treating diabetes was the discovery of insulin. The backbone of today's treatments relies on this discovery, and the patient's self-initiative and compliance. Two types of diabetes mellitus are common. Type 1 diabetes accounts for 5-10% of all cases, and Type 2 diabetes accounts for 90-95% of the diabetic population. In Type 1 diabetes, the disease requires insulin injections to maintain life, in addition it requires healthy eating and exercise. Treating type 2 diabetes may require insulin, but the disease may be controllable with oral medication, weight loss, a careful diet and a regular exercise program.
There is still no magic pill to treat diabetes. Current drugs have the potential to eliminate complications altogether, if only the patient knew when and how much to take. A program of very frequent sampling is required to provide both the rate and extent of glycemic excursions. This set of glucose measurements is absolutely necessary information to calculate the timing and amount of corrective actions needed to effectively treat Diabetes and prevent complications. The importance of blood glucose monitoring has been underscored by the results of the Diabetes Control and Complications Trial, which showed that many of the long-term complications of diabetes could be prevented by close blood glucose regulation.
However, current blood glucose tests are painful, requiring finger sticking to obtain a blood sample. They are inconvenient due to disruption of daily life and difficult to perform in long-term diabetic patients due to calluses on the fingers and poor circulation. With present technology, the average diabetic patient tests his/her blood glucose levels less than twice a day versus the recommended 4-7 times per day. Further, even the recommended testing schedule is far from sufficient to allow blood glucose normalization.
Thus with present technology, the necessary monitoring is frequently unachieved chore. The required sampling schedule cannot realistically be expected of even the most committed patients during the day and is not feasible at night. Present blood glucose monitoring methods are not automatic, chronically requiring user initiative. This system cannot therefore be relied upon to detect spontaneous hypoglycemia or other glycemic excursions. Consequently, even the most diligent patients fail to avoid severe complications. As a result $85 billion was spent in 2002 on treating Diabetes complications, including loss of sight, loss of kidney function, loss of limbs, vascular disease, heart failure, stroke, coma, and severe constant pain.
New glucose monitoring methods are needed to address these shortcomings An automatic, painless, and convenient means of continuous glucose monitoring could provide the information needed for adequate control. This would greatly reduce the complications seen in these patients and the associated health care costs of their treatment.
In order to meet the needs of continuous glucose monitoring for diabetes, the monitoring process must satisfy the following:                Require no sample preparation (the measurements occur automatically)        Be highly selective and sensitive        Provide a rapid response to changes in glucose        Provide highly repeatable/reproducible measurements        Operate with stability and low drift        
A number of different technologies have been applied to develop a glucose sensor to meet these needs. However, the most direct route to bring a successful device to the market is to develop a disposable sensor that operates in the subcutaneous tissue. This minimizes the risk of serious complications associated with a fully implanted device.
A very successful method that satisfied all of the above requirements for biosensing is enzyme based ampermetric electrode sensing. This method was intended to operate in a homogenous oxygen environment with high oxygen availability, such as a major blood vessel in the body. The employed method consumed oxygen, effectively maintaining a zero oxygen concentration at the electrode surface in order to measure oxygen. However, this approach is not directly applicable to the subcutaneous tissue.
As is often the case, the type of sensing method applied will impact the ability to achieve success in new sensing environments. Many sensing methods will perform well under in vitro or carefully controlled conditions, but will then fail to perform well in the body. Their failure has been attributed to inadequate selectivity, electrode poisoning, and insufficient glucose sensitivity.
High glucose selectivity is essential to provide an accurate measurement of glucose in the body. The selectivity of a measurement refers to the degree to which a particular analyte may be determined in a complex mixture without interference from other constituents in the mixture. In the body, there is a complex mixture that may be termed the tissue matrix in which glucose must be measured. The tissue matrix contains many constituents that are constantly changing and which may interfere with varying types of measurement approaches. The constant state of flux of the tissue matrix prevents a calibration from being established for selective measurement through a technique such as multiple regression to remove the impact of unmeasured interfering constituents of the matrix.
A full range of approaches from non-invasive to invasive are being developed in an attempt to bring a new kind of glucose sensor to market. However, while appealing, non-invasive optical measurements are generally not sufficiently selective for glucose without a detailed knowledge of the matrix being probed. The optical measurement is performed by focusing a beam of energy onto the body. The energy is modified by the tissue after transmission through the target area. A signature of the tissue content is produced by the energy exiting the tissue. The energy leaving is a function of chemical components encountered as well as thickness, color and structure of the tissue matrix through which the energy passes. In the body, the tissue matrix is constantly changing. Additionally, constant changes in the external environment, and their impact on the skin provide a non-stationary environment. This poses a severe challenge for purely optical measurements to be highly selective for glucose.
To achieve sufficient selectivity for glucose, the enzyme glucose oxidase may be employed in a semi-invasive approach. Clark and Lyons first used the strategy of combining the specificity of a biological system to achieve the necessary selectivity for glucose measurements in a tissue matrix. Glucose oxidase has a high specificity for glucose. This enzyme reduces glucose to gluconic acid and peroxide in the presence of oxygen and water.
By coupling glucose oxidase with a suitable transducer, glucose concentration may be measured by monitoring either the production of peroxide or the consumption of oxygen.
However, problems exist in the direct application of both of these approaches. Hydrogen peroxide probes often suffer from electrochemical interference by oxidizable species in a complex matrix such as encountered in the body. The electrode oxidizes these other electroactive constituents as well as hydrogen peroxide, which results in measurements with a net positive and variable error. The hydrogen peroxide if not eliminated may also have an undesirable reaction with the surrounding tissue as well as degrade the oxidase enzyme necessary for the operation of the sensor over the course of sensor operation. Additionally, unless oxygen is available in excess to the glucose being reduced by the reaction, variations in bulk oxygen will also change how much glucose is oxidized, resulting in erroneous measurements if the oxygen concentration influencing the reaction is not directly accounted for in a calibration, or prevented from impacting reaction dynamics. The problem of sensitivity to varying oxygen concentration is also present in approaches based on measuring oxygen consumption. Unfortunately, the subcutaneous tissue environment has been shown to have both a variable and heterogeneous oxygen concentration. These measurements must therefore address background oxygen variations to accurately determine the amount of oxygen being consumed, necessary for accurate glucose measurement.
Thus, to effectively utilize glucose oxidase a series of problems should be overcome. First, sufficient oxygen should be available for the reaction to proceed. Second, regardless of how glucose oxidase is coupled to a transducer, if oxygen is not available in excess, as will be the case in the subcutaneous tissue, then knowledge of oxygen concentration is needed to provide a completely selective measurement of glucose. Third, glucose oxidase should be coupled to a detector in a manner satisfying the requirements of biosensing stated above.
Much work has been done on coupling glucose oxidase to an electrode. However, problems of stability, drift, selectivity, and sensitivity must be overcome if using an electrode system. Extensive efforts have been devoted for stabilizing the electrode, minimizing the error of electroactive interference, and preventing “electrode poisoning”; however, an alternative approach is to avoid using an electrode as a transducer by selecting an optical measurement method. In the subcutaneous environment, oxygen is scarce, making an optical method that measures the result of the glucose oxidation reaction while not consuming oxygen even more desirable.
In coupling glucose oxidase with an optical transducer, an additional problem of selective sensing of oxygen is posed. Fortunately, optical means of selectively sensing oxygen are well established.